Yep, I am actually building another one today based on the other cars I have seen. I am going to do the bullet style, makes the car a hair over 1/4" thick. I did the same thing with the one wheel off and I also cambered them so only the edge of the wheel touches. My 7 year old can't do that, but he can at least tell you why all these things are important. He understands friction, drag, and so on. Of course, this is the same child that read up on the Domain Theory of Magnetism on his own and came to me with questions. Good thing he got into the gifted program here, otherwise, he would be a handful at school.
I will post up pics of the two cars we have. He originally wanted a hotrod bus. Not very aerodynamic.